This invention relates to a modified graphite fluoride consisting of graphite fluoride particles coated with an organic polymer and a method of preparing the same. The polymer-coated graphite fluoride of the invention is of use either in powder form, as solid lubricant for example, or in compacted form optionally with addition of a synthetic resin as bearings or sealing elements for example.
Graphite fluoride is a solid material in the form of white or grayish powder obtained by reaction between graphite or carbon in different form and fluorine. As typical examples of graphite fluoride, (CF).sub.n and (C.sub.2 F).sub.n are known as stable and industrially useful polymeric compounds. Generally graphite fluoride exhibits remarkably high lubricating and water- and oil-repelling properties and is excellent in resistance to various chemicals. Accordingly graphite fluoride has been used as solid lubricant in many fields and, besides, serves for releasing, water- or oil-repelling and anti-contaminating purposes. Also it is known to produce a solid body of a specific use, such as an electrolytic cell electrode, by press-shaping of a composition containing graphite fluoride as a main ingredient.
In practical applications, however, very strong water- and oil-repelling property of graphite fluoride, which is attributed to extraordinarily low surface energy of this material, offers inconvenience or difficulty in various respects. That is, this material can hardly be dispersed in water and is very low in miscibility with organic materials and poor in formability.
Regarding the use of graphite fluoride as solid lubricant, it is ideal that fine particles of pure graphite fluoride provide a continuous and closely contacting film on the applied surface, and for this reason often it is wished to disperse graphite fluoride in water without using any auxiliary material. Actually, however, graphite fluoride is practically devoid of wettability with water as demonstrated by the fact that the contact angle of (CF).sub.n for water is 145.degree., which is a very large value compared with the 100.degree.-110.degree. contact angle of polytetrafluoroethylene (PTFE) useful as solid lubricant, and therefore it is practically impossible to disperse pure graphite fluoride in water.
In view of this problem, it has been proposed to use a dispersing agent such as colloidal silica jointly with graphite fluoride. Although the use of such a dispersing agent is effective for preparation of an aqueous dispersion, there arises another problem that the content of graphite fluoride in the dispersed solid phase cannot be made so large as desired: the graphite fluoride content must be limited to about 60% by weight at the maximum. Therefore, it becomes impossible to fully utilize the favorable properties of graphite fluoride originated in the low surface energy of this material. Also it has been proposed to coat the particles of graphite fluoride with a binding material such as wax or a mixture of a binding material and a surface-active agent. In practice, however, it is very difficult to achieve uniform coating of the graphite fluoride particles by using a desirably small amount of such a coating material so as to allow the coated graphite fluoride to sufficiently exhibit its characteristic properties. Furthermore, the coating is not always stable under various conditions in the uses of the coated graphite fluoride because the coating is established merely by adsorption and adhesion, i.e. physical bonding, of the binding material onto the surfaces of the graphite fluoride particles.
In the case of producing an electrode of a primary cell by using graphite fluoride as a typical example of compacting of compositions containing graphite fluoride as the principal ingredient, it is known to press-shape a mixture of graphite fluoride and PTFE. The mixture is usually prepared by using an aqueous dispersion of PTFE obtained by emulsion polymerization of tetrafluoroethylene. Since graphite fluoride is strongly water-repelling, there is the need of suspending graphite fluoride particles in an organic solvent having strong affinity for water in advance of the addition of graphite fluoride to the aqueous dispersion of PTFE, and it is necessary to use a considerably large amount of organic solvent in order to fully wet the graphite fluoride particles. However, the use of such a large quantity of organic solvent causes coagulation of PTFE particles during mixing of the graphite fluoride suspended in the solvent with the aqueous dispersion of PTFE, so that the mixing results in formation of undesirably large agglomerates and fails to give a uniformly mixed powdery mixture. Furthermore, the large agglomerates are very tacky and, hence, are difficult to thoroughly pulverize. If the mixture containing the large agglomerates left uncrushed is subjected to press-shaping, it is very difficult to obtain a shaped body of good quality because the existence of the agglomerates becomes a significant obstacle to uniform transmission of the applied pressure and therefore is liable to produce strains in the press-shaped body.